The present invention relates generally to data storage. More specifically, a write pulse signal generator is disclosed for writing data to an optical disc.
Numerous formats exist for writing data to an optical disc including CD-R, CD-RW, and DVD. In addition, other formats have been proposed that would allow multilevel data (data that includes more than two possible information states per symbol or mark) to be written to an optical disc. As storage density increases and the mark size decreases for various optical data storage schemes, the ability to precisely control the laser waveform used to write data to an optical disc has become more important. In addition to precisely controlling the waveform, it has also become important to provide flexible control so that different waveforms for different write strategies may be supported. In general, it would be desirable if waveforms with power controlled precisely as a function of time could be reliably generated.
FIG. 1A is a diagram illustrating a CD-R laser writing waveform. The waveform begins at time t0 where the output power is the write power. The write power is maintained until a time tl, when the power is reduced to the erase power and the waveform continues until tf when the power is reduced to a low reading level power. The length of the time interval between tl and tf is determined by the length of the mark being written. The length of the mark is expressed in terms of a time interval, T, and mark lengths vary from 3T to 11T, with 3T being the shortest mark to 11T being the longest mark. The transition at tl between the write level and the erase level is programmed and does not vary with the length of the mark being recorded or with previous or future marks. The leading edge of the waveform at t0 maybe changed by an amount xcex94t that is approximately equal to xc2xcT. The leading edge is shifted by xcex94t only when the previous mark is a 3T mark, the shortest mark allowed in a CD-R system. Thus, there is some coarse control over the leading edge of the waveform when the previous mark is a 3T mark. However, control is not provided based on future marks and precise control based on previously recorded marks is not provided.
FIG. 1B is a diagram illustrating a laser writing waveform for writing a CD-R mark after a 3T mark has been recorded. The leading edge is shifted by xcex94t and the remainder of the waveform is the same.
It should be noted that, as shown in FIGS. 1A and 1B, the write power and erase power are named based on the names assigned to control lines of the laser driver. The erase power therefore does not necessarily designate a power used to erase a mark. It should also be noted that the minimum power of the laser may be a biasing power that may be designated as the reading power of the laser. The power enabling signals are labeled as write power and erase power for the purpose of designating the selected power enable line that is controlled on the laser driver. It should be recognized that these names are arbitrary and that they are only meant to designate different power levels that may be specified for a laser driver.
The waveform described above for the CD-R write strategy is one example of a standard waveform used to implement a write strategy. In general, different write strategies require different waveforms to write data. It would be useful if a single chip could be used to programmably implement multiple write strategies according to instructions received from a processor. Furthermore, what is needed for more advanced write strategies such as multi-level write strategies is a method of specifying transitions more precisely. Specifically, a method is needed for altering writing waveforms to compensate for intersymbol interference and to accommodate adaptive processing techniques that may vary the writing waveform as a result of feedback.
A system is disclosed for providing precise control of a laser writing waveform. The leading and trailing edges of the waveform are determined by previous, current, and future marks written to the optical disk. The writing waveform may be varied to write a multilevel mark within a given mark area.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication lines. Several inventive embodiments of the present invention are described below.
In one embodiment, a system and method are disclosed for generating a transition in a laser control signal at a precise point in time. A write strategy processor is configured to input a sequence of data and to determine a transition in a laser control signal from the sequence of data. The transition is specified by a number of pulse clock units and a delay. A write pulse generator is configured to input the number of pulse clock units and the delay and to generate a transition by creating a signal having a transition at the time specified by the number of pulse clock units and delaying the transition by the specified delay.
In one embodiment, a write pulse adjuster for adjusting a transition in a laser control signal by a precise amount of time includes a clock input configured to receive a clock having a clock input period. A coarse delay lock loop has a plurality of coarse delay cells each having a coarse delay period. The coarse delay lock loop locks to the clock input and the coarse delay period of the coarse delay cells is set as a first fraction of the clock input period. A fine delay lock loop has a plurality of fine delay cells each having a fine delay period. The fine delay lock loop locks to a periodic signal derived from the outputs of successive coarse delay cells. The fine delay period of the fine delay cells is set as a second fraction of the delay of the coarse delay cells. A variable delay line is configured to delay the transition by a selected number of coarse delay periods and a selected number of fine delay periods.
In one embodiment a write pulse generator for generating a transition in a laser control signal at a precise point in time includes a transition generator that generates a transition. A coarse delay line includes a plurality of coarse delay cells having coarse delay cell outputs. The transition is input to the coarse delay line. A first multiplexer having a first multiplexer output is configured to select one of the coarse delay cell outputs. A fine delay line includes a plurality of fine delay cells having fine delay cell outputs. The first multiplexer output is input to the fine delay line. A second multiplexer has a second multiplexer output configured to select one of the fine delay cell outputs. The transition is delayed by an amount determined by the selected coarse delay output and the selected fine delay output.
In one embodiment, a method of generating a transition in a laser control signal at a precise point in time includes receiving a sequence of data. A transition in a laser control signal is determined from the sequence of data. The transition is specified by a number of pulse clock units and a delay. A transition is generated by creating a signal having a transition at the time specified by the number of pulse clock units and delaying the transition by the specified delay.
In one embodiment, a method of generating a transition in a laser control signal at a precise point in time includes generating a transition. The transition is input to a coarse delay line including a plurality of coarse delay cells having coarse delay cell outputs. One of the coarse delay cell outputs is selected using a first multiplexer having a first multiplexer output. The first multiplexer output is input to a fine delay line including a plurality of fine delay cells having fine delay cell outputs. One of the fine delay cell outputs is selected using a second multiplexer having a second multiplexer output.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.